1. Field of the Invention
The present invention relates to a sublimation type thermosensitive image transfer recording medium, and a thermosensitive recording method using the thermosensitive image transfer recording medium.
2. Discussion of Background
Recently a demand for full color printers is increasing year by year. Representative recording methods for full color printers now available include an electrophotographic method, an ink-jet method, and a thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most widely employed because of its advantages over the other methods in that maintenance is easy and operation is noiseless.
In the thermosensitive image transfer recording method, a solidified color ink sheet and a receiving sheet are employed, and the color ink is transferred imagewise from the ink sheet to the receiving sheet due to the thermal fusion of the ink or the sublimation of the ink, under the application of thermal energy by laser beams or a thermal head which is controlled by electric signals.
Thus, the thermosensitive image transfer recording method can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type. The sublimation image transfer type is advantageous over the thermal fusing type in that halftone can be obtained without difficulty and image gradation can be controlled as desired. These benefits exist because a sublimable dye is in principle sublimated in the form of independent molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, through a thermal head. Therefore, the sublimation image transfer type is considered the most suitable for color printers.
The sublimation image transfer recording method, however, has a shortcoming in that its running cost is high, because in this image transfer method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet and when necessary, a black ink sheet, have to be employed in order to obtain a full-color image, with selective application of thermal energy to each ink sheet, and discarded after the recording, even though large unused portions remain on each ink sheet.
In order to eliminate this shortcoming, the following proposals have been made: (1) an equal speed mode in which an ink sheet and a receiving sheet are moved at the same speed for using the ink sheet in repetition and (2) an N-times use mode in which the running speed of the ink sheet is made lower than that of the receiving sheet so that the overlappingly used portions of the ink sheet at the first use and the second use are shifted little by little.
In the sublimation type thermosensitive image transfer recording method, the sublimation and evaporation reaction is fundamentally a reaction of zero order. Therefore, in the equal speed mode, the ink sheet cannot be used multiple times for printing because the printed image density significantly decreases as the number of printings increases, particularly in high image density areas, even though a sufficient amount of a dye for multiple printing is contained in the ink layer of the ink sheet.
In order to improve the drastic decrease in transferred image density during multiple printing, the present inventors proposed a sublimation type thermosensitive image transfer recording medium comprising a dye supplying layer and an image transfer facilitating layer in Japanese Laid-Open Patent Application 63-62866. In this recording medium, the sublimable dye discharging performance of the dye supplying layer is made greater than that of image transfer facilitating layer.
In the above recording medium, however, only a small amount of a binder resin is generally incorporated into the dye supplying layer in order to make the concentration of the dye relatively high or in order to increase the diffusion coefficient of the dye. This brings about low adhesion between the dye supplying layer and a substrate, and, as a result, the ink layer transfers in its entirety to an image receiving layer (exfoliation of the ink layer) depending on the recording conditions, for example, when high voltage is impressed.
Furthermore, when the multiple printing is conducted in the N-times use mode, an ink layer and an image receiving layer adhere to each other or friction is caused therebetween, so that improper running of the ink sheet tends to take place.